Can you decline support from another player? For example: I'm being attacked, but another one of my opponents who supports me wants to defend for me with one of his characters with renown, which would win him the game. Can I tell him I don't want him to defend for me?

Nope, you can't "Decline" support. You can block, failing to provide him the opportunity to support… but if you don't block, there's nothing (aside from title switching) you can do to prevent him from supporting.

Well, you can alway try to talk him out of it, of course - or talk the attacking player out of attacking you.

The point, though, is valid. The original defender does not have to consent to the supporting player's declaration of defenders. His only control over it is to declare his own defenders.